Now then,just received my new Insurance Docs,there's a point saying they will not pay any claim which is covered under any other policy of insurance or Trade Union membership,,or by any other policy of insurance or by Trade Union membership if this policy did not exist.

It means that, if any coverage on your policy is also covered by another policy or by potential compensation paid as a result of your being a memeber of a trade union then you must claim on those other policies. Having coverage under more than one insurance policy for any particular risk (and that can be any insured risk - not just motor insurance) is called double indemnity and obviously that is not acceptable to the underwriters. Such clauses are therefore included to prevent you from claiming twice.